Manufacture of photographic film base



June 1941-. E. K. CARVER ETAL 2,245,429

MANUFACTURE PHOTOGRAPHIC FILM BASE Filed May 18. 1937 FIGJ.

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FILTER INVENTORS BY W ATTO EYS Patented June 10, 1941 MANUFACTURE OIEZISOTO GRAPHIC FILM Emmett K, Carver and Clarence L. A. Wynd, Rochesten'N. Y., assignors to Eastman Kodak Company," Rochester," Y., a corporation of New Jersey Application May 18, 19317 ,Seria1 No. 143,302 5 Claims. (Cl. 18-57) This invention relates to a process of producing cellulose nitrate .sheeting, and more particularlyto a process of producing at hitherto unattainable speeds cellulose nitrate sheeting adapted for use as photographic film base.

As is well-known, photographic film base and 7 similar types of sheeting are generally made by depositing a thin layer of a solutionof a cellulose derivative in a volatile solvent or solvent mixture on an appropriate film-forming surface, such as the surface of a slowly rotating wheel, removing solvents therefrom until this layer has set in the form of a, film, removing the film from the surface and curing out residual solvent. For the manufacture of most types of photographic film base, particularly that used in motion picture film, it is customary to employ solutionsoor dopes of cellulose nitrate of various types. One of the most commonly employed dopes is one which may be referred to as non-coagulating, that is, one which gradually dries down into the form of a solid film by evaporation of solvent.

In view of the steadily increasing demand for cellulose derivative sheeting for photographic and other purposes, numerous attempts have been made to increase the speed .andoefficiency of film-forming operations. For example, it has been proposed to employ a coagulable dope and coagulate such dope in a humid atmosphere. While some improvement in casting speed is thus obtained, the increase in speed thus far obtained has been of only moderate proportions. Furthermore, when attempts have been made to increase the speed, as for example, by adjusting the amount of moisture, wheel temperature, air temperature, dope composition or other factors, numerous. difiiculties have arisen-and, until the advent of the present inventiomit hasnot been possible to accomplish any marked advance in speed .without such asacrifice in quality ,of the finished product as to render it substantially unfit for its intended uses.

This invention has as its principal .object ,to provide a process for producing cellulose nitrate sheeting adapted for use as photographic .film

base and similar purposes at hithertoounattainable speeds. A further object is to 'increasethe speed and efiiciency of manufacture ofcellulose nitrate sheeting by that method which involves coagulation of a coagulable dope in the presence of a humid atmosphere. A specific object is to produce cellulose nitrate film base adapted for use in the manufacture of motion picture :film and other purposes by .the coagulating method more percent greater than the speeds hitherto obtainable in the. production of satisfactory film base. Other objects will appear hereinafter.

These objects are accomplished by the following invention which comprises the discovery that if one employs acertain specific type of coagulable dope, namely, a dope composed of cellulose nitrate having a nitrogen content of 11.0% to 12.1%, dissolved in methyl alcohol to which a cosolvent such as acetone has been added in an amount sufiicient to give a clear, homogeneous solution, and coagulates this dope by means of steam, film or sheeting of satisfactory qualitycan be produced at greatly increased speeds. We

have worked with a wide variety of dopes of ,varying composition in the manufacture of photographic film and have finally discovered that only by the use of a dope which can be coagulated or brought into a condition in which it may be satisfactorily stripped from the filmiorming surface while still containing a relatively large amount of solvent, can appreciably increased speeds be attained. We have found that ,the ability of cellulose nitrate dopes to be cast into clear usable film or sheeting of the desired properties varies,,not only ,Wit-hthe nitrogen content of the cellulose nitrate, but also with the amount of moisture which is almost invariably present under actual operating conditions.

Generally speaking, we have found thatcellu- ,losenitrates havingra nitrogen content of not over 12.05%, for example, are soluble in substantially pure methyl alcohol, but that as the nitrogen Vcontent is increasedbeyond this figure, increasing proportions of ,acetone or other cosolvent .mustbe addedto obtain a solution sufficiently clear and homogeneous for practical film- ,makingpurposes. By employing arsrnallamount eofacetone, for example, we may employ acellulose,nitrate having a nitrogen content as high ,as-,l2.1 Onthe otherhand, we have discovered athatonemust not employ a solvent mixture which too easily dissolves the cellulose nitrate or --one cannot coagulate the composition by means -at speedsup to approximately .rfive hundred or55 -By .theiterm i co solyent .of sufiicientmoisture to attain the desired increase in castingspeedwithout the production of 'undesirablehaze, opacity, or other undesirable effects in the resultingfilm. a

As will. be .more fully set forth hereinafter, we have found that due to the. small amount of moisture which almost invariably finds its way into thefilm-forming composition, it is necessary ztoiaddua. co-solventgfo'r theadditional purpose of .keeping. thesolution clear under these conditions. .as here used, .wereifer to a substance which is, either itself a good solvent for the cellulose nitrate, or a substance which increases the solvent power of the methyl alcohol for the cellulose nitrate. After extensive work with compositions of this nature, we have found that those compositions in which the cellulose nitrate has a nitrogen content of 11.0% to 12.1% and is dissolved in a solvent mixture composed of anywhere from 100% to 86% methyl alcohol and containing from to 14% acetone as co-solvent are peculiar in their ability to be stripped from the film-forming surface while still containing relatively large amounts of solvent. However, we have also found that all of such compositions are not satisfactory for this purpose, since, as will be more fully set forth hereinafter, they are not all susceptible of coagulation into satisfactory films in the presence of steam at increased speeds.

The nature of these solutions or dopes will be more fully understood by reference to the drawing, in which;

Fig. 1 is a graphical representation of various cellulose nitrate compositions satisfactory for use in our process.

Fig. 2 is a diagrammatic sectional elevation of a conventional form of device adapted for the manufacture of film or sheeting from the compositions illustrated in Fig. 1.

Referring to Fig. 1, the area lying underneath curve A designates compositions all of which are coagulable by means of moisture in accordance with our invention to produce clear, transparent film or sheeting satisfactory for photographic and other purposes. The areas lying under curves B and C, respectively, and the area lying between curves C and D, represent preferred film-forming compositions, for use in accordance with our process. It is to be understood from Fig. 1 that not all compositions in which the cellulose nitrate has a nitrogen range from 11.0% to 12.1% dissolved in a solvent mixture containing from 100% to 86% methyl alcohol and from 0% to 14% of acetone are satisfactory for our purpose, but only those compositions which fall Within the area lying underneath curve A. We have found, for example, that when one attempts to employ compositions lying appreciably above curve A under such conditions that an increase in casting speed is obtained, the product has a definite tendency toward the development of haze or opacity. This tendency is more pronounced, the more the film-forming process is speeded up as, for example, by an increase in the amount of moisture present in the coagulating atmosphere. In other words, while under some circumstances it may be possible to produce usable film from some of the compositions lying above the curve, it is impossible to produce satisfactory products and at the same time attain appreciable increases in speed over those heretofore attainable, which is the primary purpose of the instant invention.

On the other hand, we have found that compositions falling within the area underneath curve A may all be coagulated at markedly improved speeds, or to express it in another way, the length of time during which such compositions must remain on the film-forming surface before satisfactory stripping can be attained is reduced from approximately ten minutes (the time required for standard dopes of either the coagulating or noncoagulating types) to as little as 2.4 minutes or less under optimum conditions for a. given dope.

tographic film must be reduced to at least about 25% before satisfactory stripping can be obtained,

'- the compositions of our invention may be satisfactorily stripped while containing anywhere from 50% to 75% solvent. While an almost infinite variety of cellulose nitrate compositions have been known and used for many years in i the film making art and the industry has been This marked decrease in stripping time is diconstantly seeking a method whereby casting speeds could be increased, not until the advent of the present invention has it ever been recognized that the employment of a coagulable dope of the character herein set forth could bring about this highly desirable result. In other words, an increase of anywhere from a few percent to as much as 400% or 500% or more in the speeds hitherto attainable represents a totally inobvious and unexpected result.

From the above description it will be seen that our invention resides in the fundamental discovery that when certain types of cellulose nitrate dopes in which the cellulose nitrate is dissolved in a. solvent to which a co-solvent has been added in a sufficient amount, depending upon the nitrogen content of the cellulose nitrate and the amount of moisture present, to produce a clear, usable film-forming solution,.such compositions may be coagulated by means of moisture and brought to a condition in which they may be satisfactorily stripped from the film-forming surface while still containing relatively large amounts of solvent and therefore in much less time than has hitherto been required when employing prior art dopes.

We have referred above to the use of a cosolvent for maintaining the dope in a clear, homogeneous satisfactorily coagulable condition. By the term co-solvent we mean an agent which promotes solution of the cellulose nitrate in the main solvent (methyl alcohol) and renders the dope less subject to incipient coagulation under the conditions of operation when water is unavoidably present therein. While we have found acetone to be the most satisfactory agent for this purpose, it is within the scope of our invention to employ other co-solvents such as methyl ethyl ketone, diethyl ketone, ethyl acetate, butyl acetate, ethyl ether, methyl ethyl ether, isopropyl ether, dioxane, ethyl cellosolve, methyl cellosolve, dimethyl cellosolve and methyl cellosolve acetate.

In producing sheeting in accordance with our invention, a cellulose nitrate solution or dope of a composition falling within the area lying underneath one of curves A, B and C, of Fig. 1, may be coated or cast into the form of a film by means of a device such as that illustrated in Fig. 2. In this device, numeral I designates a conventional type of film-forming apparatus comprising the casting or coating wheel 2. The wheel assembly is carried by base 5 and surrounded by wheel casing 6 provided with air inlet conduit 1 and outlet conduit 8.

The numeral 9 designates a conventional type of dope hopper provided with adjustable gate member ID for regulating the thickness of the dope stream which flows therefrom, adjustment being by means of thumbscrew ll, attached in known manner to the gate member and threaded through one .wall of, the hopper,

Inlet conduit 1 is provided with air filter l2, steam jet I3 for (supplying thefdesired degree of moisture to the wheel casing, and with heater M for heating theair supplied thereto. Circulation of the heated moisture-laden air through conduit 1, wheel casing 8. and conduit 8 is accomplished by means of an appropriate air pumping or circulating device (not shown) connected to conduit 8, the solvent-laden air being passed to a. suitable solvent recovery system (not shown). Numeral l designates a stripping roll to facilitate removal of the coagulated film F from the surface of wheel 2.

The operation of the device will be apparent on inspection. As the Wheel 2 rotates slowly in the direction indicated, dope is fed from the hopper 9, the thickness of the dope stream being regulated by adjustment of. gate member ID to give a film of appropriate eventual thickness, say.

have any deleterious action on the film.

As soon as the film has set sufliciently to be satisfactorily removed from the wheel, it is stripped therefrom at stripping roll 15 and may then, in accordance with standard practice, he passed to an appropriate curing chamber where residual solvent is cured out and the film subjected to any other desired treatment.

As indicated above, the compositions the use of which we have found result in increased production speeds are those in which the nitrogen content of the cellulose nitrate lies within the ranges indicated in Fig. 1 and in which the solvent has the indicated composition, that is, a solvent composed of at least 86% methyl alcohol and not more than 14% acetone or equivalent co-solvent. In general, any composition falling Within the areas lying under curve A of Fig. 1 of the drawing is satisfactory and the use of such a composition results in an improvement in casting speed. However, the compositions falling respectively underneath curves B and C, and between curves C and'D constitute preferred species. Examples of the above compositions are as follows:

Parts by weight The cellulose nitrate employed in our compositions may be one prepared by any of the standard methods, but we prefer to employ a nitrate having .a nitrogen content ranging from 11.7% to 12.05% and prepared as described in the .sure good solution is in .this case about 1%.

U. S. .Patent to R. H. Talbot, 2,118,275, issued May 24, 1938. In fact, a preferred film-forming composition is composed of a nitrate of this nitrogen range prepared in accordance with the procedure of that application, dissolved in methyl alcohol containing as little acetone as is consistent with obtaining a clear, usable solution in view of whatever moisture may be present.

The ideal composition from the standpoint of casting speed in accordance with our invention would be one in which the cellulose nitrate has a nitrogen content of about 11.7% .to 12.05% as above indicated, dissolved in anhydrous methyl alcohol and containing no plasticizer. Under the conditions met with in plant practice, however, this ideal of employing an anhydrous solvent is usually unattainable, due to the fact that small amounts of water find their way into the composition, either because of the fact that methyl alcohol is generally contaminated with small amounts of water, or because moisture is introduced into the dope from imperfectly dehydrated cellulose nitrate or even from humid air during the preparation of the solution. Because of the tendency of such a composition to coagulate or gel in the presence of moisture the introduction of even small amounts of water will cause incipient coagulation and give solutions which are characterized as grainy by reason of the presence of minute coagulated particles of cellulose nitrate suspended therein. We have, therefore, found that it is necessary to employ a small amount of acetone or other co-solvent mixed with the methyl alcohol to increase the solubility of the cellulose nitrate in the solution and, :by thus preventing incipient coagulation, to maintain the dope in a clear, usable condition. It is necessary, however, to limit the amount of acetone, since as the acetone content of the solution is increased, not only is the time which it is necessary for the film to remain on the filmforming surface increased, thereby tending to defeat the primary object of the invention, but there is also an increased tendency toward haze -or opacity in the product. When acetone is used in larger amounts than those indicated, that is, so as togive a composition lying to the right and above the curve A of Fig. 1, roduction of clear films at greatly increased speeds is out of the question. The water content of cellulose nitrate solutions prepared in large scale manufacturing equipment, however, rarely exceeds 1%, and the minimum amount of acetone necessary to in- In practice,3-4% of acetone may be employed and the dope may contain up to 14% acetone, depending on the amount of water present and still'be satisfactory for our purpose. The particular amount of acetone will therefore vary, not-only with the water content of the dope, but

also with the particular nitrate selected, the

speed it is desired to attain and other factors.

Thus, no hard and fast rule can be laid down as to the particular amount to be employed. In any event the maximum amount of acetone for any given composition, as indicated by the graph of Fig. 1, is adequate to compensate for the pres ence of such small amount of water as may be encountered under ordinary manufacturing conditions.

Although optimum coating speeds are obtained in the absence of a plasticizer, it is preferable to employ a small amount of a plasticizer such as cam-phor or :triphenylphosphate to lessen the .in-

fiammability of the film. In general, it may be said that the greater the amount of plasticizer employed, the greater will. be the stripping time, that is, the time required to bring the film into a condition in which itmay be satisfactorily removed from the film-forming surface.

While it is fundamental to our process to coagulate the dope by means of moisture, the specific concentration of Water vapor in the film-forming atmosphere may vary widely, depending upon the composition of the dope, the speed desired, the coagulating temperature and other factors. In general, it may be said that the higher the concentration of moisture, the higher the casting speed. When employing dopes having compositions such as indicated above, we have found that the moisture may run anywhere from a few grains to as much as 600 grains of water per pound of air in contact with the coagulating film. The wheel temperature, that is, the temperature at which the film is undergoing coagulation mayvary considerably, depending upon the composition of the dope employed and various other factors, and may range anywhere from room temperature to any temperature below that which would deleteriously affect the film. For the compositions indicated above an air temper- .ature of about 150 F. (whichgives a wheel temperature of about 90 F.) is satisfactory. It will,

of course, be understood that in general, the

lower the temperature, the lower will be the casting speeds obtainable, but even at low temperatures greatly increased casting speeds (as compared to those obtainable with known film-forming dopes employed under the same temperature conditions) are obtainable by using our process.

As indicated above, after the film is stripped from the film-forming surface-it may be cured in any convenient manner, as by conducting it through an appropriate curing device. Inasmuch as our process is concerned only with the film-forming operation per se, no discussion of such subsequent operations is necessary.

While we have found it convenient to illustrate the dope compositions, the use of which we have found make possible great increases in casting speed, by reference to the area lying to the left of an underneath curve A of Fig. 1, it will be understood that this area may, under some circumstances be shifted slightly to the right of its indicated position by a change in various factors entering into our process, as for example, a change in the amount of moisture employed in the coagulating atmosphere, the coagulating temperature, the amount of plasticizer present in the dope and other factors.

One further point should be mentioned in connection with the composition of the above-mentioned film-forming solutions or dopes and this is the fact that such dopes tolerate the presence therein of fairly considerable amounts of butyl alcohol which apparently does not appreciably affect the coagulating qualities of the composiiion when employed in accordance with our invention. As is well-known, butyl alcohol is frequently used to dehydrate cellulose nitrate after the washing which normally follows the nitrating operation and the dehydrated cellulose nitrate containing as much as 40% or more, based on the dry weight of the nitrate, of butyl alcohol may be employed with excellent results in the dopes above described. It is therefore to be understood that the broad scope of our invention includes compositions in which cellulose nitrate of the indicated nitrogen range is dissolved in a solvent mixture, the essential solvent portion of which consists of methyl alcohol and one or more co-solvents, the balance comprising butyl alcohol derived from the alcohol-wet cellulose nitrate or elsewhere, plasticizers and/or other appropriate ingredients.

From the above description it will be seen that our invention represents an outstanding advance in the art of film and sheeting manufacture. Those skilled in this art will readily appreciate the fact that even a small reduction .in stripping-time from the customary 10 or 12 minutes represents a very considerable saving in time, money and materials. A reduction in stripping time of anywhere from one to six minutes or more below the customary figure obviously represents an enormous increase in film production speeds and a resultant increase in the economy and general efliciency of the whole filmmaking process. Our invention is all the more remarkable in view of the fact that cellulose nitrate compositions of many different types have been known and used in film manufacture for decades. For years the film-making industry has been seeking a method of increasing production speeds without adversely affecting the prodnot, but not until the advent of the present invention has it been recognized that by the simple expedient of employing certain specific dope compositions as herein described could this highly desirable and valuable result be attained.

What we claim and desire to secure by Letters Patent of the United States is:

1. A high speed method of making sheeting adapted for use as photographic film base, which comprises depositing in the form of a film on a film-forming surface a solution composed of cellulose nitrate having a nitrogen content between 11.0% and 12.1% dissolved in methyl alcohol containing not over about 14%, based on the weight of solvent mixture, of a co-solvent, which solvent mixture will give a clear, homogeneous solution capable of coagulating or setting up by means of water Vapor while containing at least as much solvent as the film is to contain at stripping, to produce a clear, transparent film, said solution having a composition falling within the area underneath curve A of the attached drawing, coagulating said solution by means of water vapor While containing at least as much solvent as the film contains at stripping, removing the film from the film-forming surface while still containing about 50-75% solvent, and then curing out the residual solvent.

2. A high speed method of making sheeting adapted for use as photographic film base, which comprises depositing in the form of a film on a film-forming surface a solution composed of cellulose nitrate having a nitrogen content between 11.7% and 12.05% dissolved in methyl alcohol containing not over about 13% based on the weight of solvent mixture, of a co-solvent, which solvent mixture will give a clear, homogeneous solution capable of coagulating or setting up by means of water vapor while containing at least as much solvent as the film is to contain at stripping, to produce a clear, transparent film, said solution having a composition falling within the area underneath curve A of the attached drawing, coagulating said solution by means of water vapor while containing at least as much solvent as the film' contains at stripping, removing the film from the film-forming surface while still containing about 50-75% solvent, and then curing out the residual solvent.

3. A high speed method of making sheeting adapted for use as photographic film base, which comprises depositing in the form of a film on a film-forming surface a solution composed of cellulose nitrate having a nitrogen content of 11.6% dissolved in methyl alcohol containing not over about 7%, based on the weight of sollating said solution by means of water vapor while containing at least as much solvent as the film contains at stripping, removing the film from the film-forming surface while still containing about 50-75% solvent, and then curing out the residual solvent.

4. A high speed method of making sheeting adapted for use as photographic film base, which comprises depositing in the form of a film on a film-forming surface a solution composed of cellulose nitrate having a nitrogen content of 12.0% dissolved in methyl alcohol containing not over about 12% based on theweight of solvent mixture, of a co-solvent, which solvent mixture will give a clear, homogeneous solution capable of coagulating or setting up by means of water vapor while containing at least as much solvent as the film is to contain at stripping, to produce a clear, transparent film, said solution having a composition falling within the area underneath curve A of the attached drawing, coagulating said solution by means of water vapor while containing at least as much solvent as the film contains at stripping, removing the film from the film-forming surface while still containing about -75% solvent, and then curing out the residual solvent.

5. A high speed method of making sheeting adapted for use as photographic film base, which comprises depositing in the form of a film on a film-forming surface a solution composed of cellulose nitrate having a nitrogen content of 11.9% dissolved in methyl alcohol containing not over about 11% based on the weight of solvent mixture, of a co-solvent, which solvent mixture will give a clear, homogeneous solution capable of coagulating or setting up by means of water vapor while containing at least as much solvent as the film is to contain at stripping, to produce a clear, transparent film, said solution having a composition falling within the area underneath curve A of the attached drawing, coagulating said solution by means of water vapor while containing at least as much solvent as the film contains at stripping, removing the film from the filmforming surface while still containing about 50-75% solvent, and then curing out the residual solvent.

EMMETT K. CARVER. CLARENCE L. A. WYND. 

